I think we should also prevent $force when $current_site->path == $blog->path && $current_site->domain == $blog->domain. When this site disappears, the network admin will likely not work, and startup might fail.

Ideally we need to make this more explicit, such as denying the ability to delete these blogs in the UI (at the cap level, perhaps). Perhaps a better API distinction needs to be made with is_main_site(), the root domain/path, and blog_id 1, which can all be different.